Documentation: Add diff.<driver>.* to config

Documentation: Add diff.<driver>.* to config

Although the gitattributes page contains comprehensive information
about these configuration options, they should be included in the
config documentation for completeness.

It may be better to rename the "driver" in "diff.<driver>.*" to
something like "content type" or "file type", but for now, let's keep
it consistent across this part of the documentation and the original
description in the gitattributes documentation.

Helped-by: Jakub Narebski <jnareb@gmail.com>
Helped-by: Michael J Gruber <git@drmicha.warpmail.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>